In the development of new receivers for television, more advanced signal processing techniques will be implemented in the circuitry of the video processing section. It has been proposed that such advanced video signal processing can be carried out by high-speed two-dimensional (2-D) spatial filtering or by three-dimensional (3-D) temporal filters. See for example, "Digital Signal Processing in Television Receivers" by M. J. J. C. Annegarn, A. H. H. J. Nillesen, and J. G. Raven, Philips Tech. Rev. 42, No. 6/7, 183-200, Apr. 1986. Newer receivers will display pictures that have higher pixel resolution. For what has been termed High Definition Television (HDTV), if the processing is done in real-time, that is, at the same rate as the effective sampling rate of the picture, processing rates as high as 40 million pixels/sec would be required. Methods of deriving high-speed filtering structures cast in the form of practical apparatus, constructed entirely from conventional devices and components, are of current interest not only for application in the area of advanced television systems, but also in the area of biomedical and industrial video applications. Such a method of deriving practical structures together with the characteristic high-speed two-dimensional video signal processing apparatus that results from the application of the method is the subject of the present invention.
In the past analog signal processing techniques such as noise coring, edge peaking, and comb filter separation of luminance and chrominance signals have been based on one-dimensional time domain approaches implemented as simple Finite Impulse Response (FIR) signal processing structures. These have been quite limited as to the type of signal processing and enhancement operations that could be performed by the characteristic type of apparatus that resulted from following that approach. In general, an FIR filter structure will require a higher order signal processing structure than for an equivalent Infinite Impulse Response (IIR) filter structure; hence, the complexity of apparatus that would be required to embody such an FIR structure will be correspondingly greater. The method of the present invention overcomes these limitations by making possible the derivation of true 2-D signal processing apparatus with the real-time capability inherent in analog devices and components, allowing the implementation of the gamut of 2-D filtering techniques in either IIR or FIR structures.
The direct application of techniques for filtering 2-D data, known from the field of Mathematical Image Processing Theory, has been hampered by the difficulty in developing digital filter structures that can be embodied in the form of practical conventional digital hardware apparatus that can operate at the high data rates required for real-time video signal processing. See for example, "High-Speed Architectures for Digital Image Processing", by A. N. Venetsanopoulos, K. M. Ty, and A. C. P. Loui, IEEE Trans. on Circuits and Systems, Vol. CAS-34., No. 8, 887-895, Aug. 1987. Considerations of hardware complexity, physical size of apparatus, power consumption, and economical manufacture are all of vital importance in any practical signal processing apparatus intended for use in consumer products. The present invention overcomes these difficulties by introducing a method and an apparatus in which the two-dimensional s,z transform is used to derive 2-D signal processing structures for performing 2-D signal processing in accordance with given specifications. These given specifications may be those that arise out of the need or desire to implement either well-known or newly derived Image Processing techniques. Those embodiments of 2-D signal processing apparatus derived by the method of the present invention are comprised of line delays, analog summing amplifiers, analog inverting amplifiers, analog integrators and passive components. The use of analog devices make these embodiments inherently capable of real-time operation while remaining practical in light of the above mentioned considerations.
Recently, motion adaptive digital filters have been proposed for use in high-definition television video signal processing. These can be regarded as common 3-D FIR filters, for the implementation of techniques for video signal processing. They require delays of one or more field periods, such delays being accomplished by means of frame-stores. Since pixels in separate fields are combined, this type of signal processing is referred to as temporal filtering and can only be performed on those pixels for which no motion in the scene of the picture being displayed has occurred between fields. Thus the development of such a filter is complicated by the necessary inclusion of circuitry that implements a motion detection algorithm. The embodiments of apparatus arising out of the present invention do not require analog to digital (A/D) and digital to analog (D/A) converters, often used in conjunction with analog pre-filters and post-filters, to convert video signal data from analog raster scanned form to sampled digital data for processing; nor do they require expensive frame-stores or motion detection circuitry.
Other methods of performing real-time 2-D signal processing have been based on elaborate algorithms such as the Burt Pyramid which separates an image into a number of 2-D spatial frequency bandpass images. See for example, "A Two-Dimensional Real-Time Video Pyramid Processor", by J. H. Arbeiter and R. F. Bessler, RCA Review, Vol. 47, 3-31, March 1986. This method, although capable of real-time operation, has the disadvantage of greater complexity relative to the present invention, due to the need to process multiple bands and to generate a set of component images. When embodied in digital hardware, a large amount of circuitry is required along with the need for A/D, and D/A converters.
Aspects of the present invention have been described in the following publications:
1. "Two-Dimensional Analog Filters: A New Form of Realization", by M. A. Sid-Ahmed, IEEE Trans. on Circuits and systems, Vol. 36, No. 1, 153-154, Jan. 1989.
2. "Hardware Considerations for the Considerations for the Realization of 2-D Analog Filters for Processing TV Images in Real Time", by H. J. Kaufman and M. A. Sid-Ahmed, IEEE Trans. on Circuits and Systems, (in press).